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Graphene could make 'perfect' solar cells

A new device that combines graphene with special metallic nanostructures could lead to better solar cells and optical communications systems. That is the claim of researchers in the UK who have measured a 20-fold enhancement in the amount of light captured by graphene when it is covered by such nanostructures. The work provides further evidence that the material might be ideal for making photonics and optoelectronics devices, despite the fact that it does not have an electronic bandgap.

Graphene is a sheet of carbon atoms arranged in a honeycomb-like lattice just one atom thick. Since its discovery in 2004, this "wonder material" has continued to amaze scientists with its growing list of unique electronic and mechanical properties. Some believe that graphene could find uses in a number of technological applications – even replacing silicon as the electronic industry's material of choice. This is because electrons whiz through graphene at extremely high speeds, behaving like "Dirac" particles with no rest mass.

Graphene also shows great promise as a candidate for photonics applications – especially optical communications, where speed is an issue. The material has an ideal "internal quantum efficiency" because almost every photon absorbed by graphene generates an electron-hole pair that could, in principle, be converted into electric current. Thanks to its Dirac electrons, it can also absorb light of any colour and has an extremely fast response to light. The latter suggests that it could be used to create devices that are much faster than any employed in optical telecommunications today.

Drawbacks addressed

Researchers have also already shown that they can make basic solar cells, light-emitting devices, touch screens, photodetectors and mode-lock ultrafast lasers from the material. However, there are, of course, drawbacks: graphene's "external quantum efficiency" is low – it absorbs less than 3% of the light falling on it. Furthermore, useful electrical current can only be extracted from graphene-based devices that have electrical contacts with an optimized "asymmetry" – something that has proven difficult to achieve.

Now, researchers at the University of Cambridge and the University of Manchester may have solved both these problems by pairing up graphene with plasmonic nanostructures. These are metal devices that enhance local electromagnetic fields in a material by coupling incoming light with electrons on the surface of the metal. The nanostructures are fabricated on top of graphene samples to concentrate the electromagnetic field in the region of the material where light is converted to electrical current, so as to dramatically increase the generated photovoltage.

The team, which includes Manchester's Andre Geim and Kostya Novoselov, winners of the 2010 Nobel Prize for Physics for their discovery of graphene, started out by preparing samples of graphene using the now-famous "sticky tape" method. This involves mechanically shaving off single layers of graphene from a block of graphite. The researchers then made two-terminal electronic devices from the material by forming contacts made of titanium and gold on the graphene using electron-beam lithography. Next, various plasmonic nanostructures were assembled close to the contacts.

Highest efficiency so far

The new devices have an external quantum efficiency of almost 50%, the highest value to date for graphene, says team member Alexander Grigorenko of Manchester. This boosts the light-harvesting capacity of graphene by more than an order of magnitude compared to its non-contacted counterpart, without sacrificing its speed. "If the plasmonic nanostructures we employed were optimized, it should be possible to realize perfect light-to-current conversion, where every photon falling on graphene is converted into current," he told physicsworld.com. "This is exactly what the solar cell industry is waiting for."

Furthermore, the problem of creating contacts with the desired asymmetry is addressed through the use of titanium and gold in the device.

"Our work is the first step towards 'perfect' photodetectors and solar cells because we have shown that plasmonics helps graphene convert light into electricity with ideal efficiency," says Andrea Ferrari, who led the Cambridge effort in the collaboration. "Optimizing light interaction and photovoltage generation in graphene will be key for a range of applications, such as solar cells, imaging and telecommunications."

Profusion of charge carriers

Graphene could also be a viable alternative to conventional plasmonic and nanophotonic materials, he added, because it has many advantages over these materials. It can absorb light over any wavelength in the electromagnetic spectrum from the ultraviolet to visible and far-infrared wavelengths, which means there is no need for bandgap engineering; and it can confine this light into unprecedented small volumes. The profusion of charge carriers in graphene and the fact that researchers can now produce the material in large quantities and over large areas means that it could outperform all existing semiconductor technologies in applications as diverse as photodetectors, tunable ultrafast lasers and imaging, claims Ferrari.

"Graphene seems a natural companion for plasmonics," adds Grigorenko. "We expected that plasmonic nanostructures could improve the efficiency of graphene-based devices, but did not expect that the improvements could be so dramatic."

Spurred on by its new results, the team now plans to study how light interacts with graphene in more detail. The researchers also hope to optimize their plasmonic nanostructures, for example by exploiting coupled or "cascaded" plasmon resonances that could further enhance the photovoltage generated. "We might also be able to increase light absorption even more by employing several layers of graphene, something that could lead to a 100-fold enhancement of the photovoltage," states Ferrari.

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11 comments

Hype

I am now retired and all those budding 'young things' out there often get excited over announcments like this.We have all got over enthusiastic when young but what happened to superconductors,engines made from ceramics and others that have fallen by the wayside.This will probably go the same way so don't hold your breath waiting for super efficient solar panels etc.I reckon a good comedy show now would be to repeat on BBC Tomorrows World?

I am now retired and all those budding 'young things' out there often get excited over announcments like this.We have all got over enthusiastic when young but what happened to superconductors,engines made from ceramics and others that have fallen by the wayside.This will probably go the same way so don't hold your breath waiting for super efficient solar panels etc.I reckon a good comedy show now would be to repeat on BBC Tomorrows World?

What a cynical mind you are.!..Only joking, I am with you on this. Graphene seems to create so much noise for nothing and it seems to grab all the cash around starving other good work these days. Judging by the way it is going, Graphene rush will stay for another several years before they decide it is definitely a superconductor.

I do hope that this graphene tempest would not go the way of the high-temperature superconductors. Of course, I do not deny that graphene with its Dirac-point leading to interesting properties, is a unique material to work with.

I tend to agree. The ease of use and production of graphene is one of the benchmarks here. The HTSC hype is always hindered by the complexity of actually making the superconductor and then having its properties tested with a new lattice element or configuration. We're not trying out different "flavors" of graphene, we're trying out different physical configurations, as anyone does with a new material. At the very least this research will get industry excited and in a few years the funding will diversify as industry research cash advances the industry. Don't count out graphene just yet.

Impolite Criticism

I may be naive, but the claims of two Nobel laureates should be given more respect than was expressed in some of the comments. Unlike cold fusion, these findings are reproducible. Improved solar panels will aid in the struggle to free society from expensive, damaging, and terrorist-ridden oil.

I may be naive, but the claims of two Nobel laureates should be given more respect than was expressed in some of the comments. Unlike cold fusion, these findings are reproducible. Improved solar panels will aid in the struggle to free society from expensive, damaging, and terrorist-ridden oil.

Nobel = more respect? Ahem...didn't Barack Obama receive a Nobel prize? For doing what, exactly? Enough said.Not enough said. This is inappropriate and demeaning to the spirit of this blog. Go drink some tea.

Not enough said. This is inappropriate and demeaning to the spirit of this blog. Go drink some tea.Do blogs have spirits? Perhaps alcoholic ones expressed in frivolity and disrespect for your "superiors".

Personally, I feel that Aristotle and the 19th century's medical research speak well for challenging them constantly.

Nobel Prizes

Distinguished prizes such as the Nobel Prize appear to me to be given in areas such as "Peace" to people who are recognized for doing exceptionally good things or for advocating good ideas in ways that unusually influence many many others to do or think good things. However, I do not believe that I or others are naive in recognizing, honoring, and being proud of all Nobel Laureates for their contributions and, particularly in the sciences, many of those awards are based on truly seminal work, often done years before the award, that retrospectively has significantly influenced our lives, our technologies, and our sciences in so many amazing ways. So, yes, I am interested and excited by this short article, in large part because it does involve people whose work has already been so respected by others. For the first cut, reputation is almost always a useful factor as well as is some degree of general cautionary skepticism.